I have used E6000 on nearly every part of nearly all my builds. It has never stained any parts of my armor and has never broken down over two years of use.
Let me also add, these types of lessons are hard learned, but we all make them. Do not get discouraged by this. You will come out of this an experience an armor builder and better for it.
Agreed. That is not much support given the stress the joint takes. I would consider a piece that gives at least 2" of support to each side of the joint, preferably 3 to 4 inches.
Nice work on filling the gap with slurry. Sorry its tearing.